1. Field of the Invention
The present invention concerns a process for converting nylon 6, nylon 6,6 or mixtures thereof to a single monomer species, adipic acid. More specifically, the invention concerns a process comprising reacting nylon 6 and/or nylon 6,6 with an aliphatic monocarboxylic acid of the formula, H.sub.3 C--(CH.sub.2).sub.n --C(.dbd.O)--OH, where n is 0 to 4 to form alkylamido monomer species, and oxidizing the alkylamido monomer species to adipic acid.
2. Description of Related Art
There is waste associated with the manufacture of any man-made article and the disposal thereof after it has served its useful life. For purposes of both economy and environmental considerations it is desirable to recover as much of the value of the material as possible. Although incineration may be employed to recover some value as fuel, recovery of the material from which an article was originally fashioned so that it may be reused is a worthwhile objective.
Nylon 6 and nylon 6,6 form the vast majority of polyamides found in commercial use worldwide. Distinguishing between these two polyamides requires sophisticated test methods. There is a great need for a method to process both polymers without separating them from each other to yield an intermediate material that can be reused to produce new nylon.
Numerous methods to depolymerize nylon are known. Nylon 6 is routinely depolymerized under the influence of steam to caprolactam for reuse by nylon 6 producers. Other methods have been reported, such as hydrolysis of nylon 6 to aminocaproic acid in aqueous barium hydroxide solutions as described in Kowolik et al., published British patent application GB 790,503. Processes for depolymerizing nylon 6,6 for reuse are also known. Monet, U.S. Pat. No. 3,069,465 describes a method for depolymerizing nylon 6,6 with aqueous sulfuric acid, recovering adipic acid and, after neutralization, hexamethylene diamine. Habermann et al., German published patent application DE 4,219,757 describes a method for the depolymerization of nylon 6,6 by aqueous base solution, separation of the diamine by extraction and converting the diacid salt into the corresponding diacid by electrolysis.
Recovery of useful monomers from mixtures of nylon 6 and 6,6 are less well known in the art. Moran, U.S. Pat. No. 5,266,694 discloses the recovery of caprolactam and hexamethylene diamine from a nylon 6/6,6 mixture under the influence of steam in the presence of a basic catalyst. McKinney, U.S. Pat. No. 5,302,756 describes the ammonolysis of mixed nylons to produce a mixture of monomers suitable for conversion to hexamethylene diamine.
Depolymerization of nylon 6,6 or mixtures thereof with nylon 6 generally produces multiple monomer products which must be separated from one another or subject to further modification prior to reuse. Another hindrance to efficient production of monomer species from waste nylon is the separation of the nylon fraction from composite materials, such as carpets. This is usually accomplished mechanically, adding to the expense.
There is high value in a process that can treat composite materials containing mixed nylons with a solution of a reagent that can both separate the nylons from the foreign material, depolymerize the nylons and convert the depolymerized nylon into a single monomer product suitable for reuse in producing new nylon.